Turbocharging and precombustion chamber spark plug internal combustion engine, ignition emthod and application

ABSTRACT

The invention concerns an internal combustion engine with at least one engine member, the engine member including a combustion chamber ( 4 ) of a combustible mixture with fuel components and oxidants, an ignition system of the combustible mixture by an igniter ( 7 ), sequential let-through devices for the fuel and oxidant components and for the combustion products, the engine being of the supercharging type by boost pressure of the oxidant components upstream of the engine member. According to the invention, the ignition system includes a closed head ( 6 ) ( 12   a ) substantially spherical enclosing the igniter in a precombustion chamber, the head including a set of orifices ( 5 ) intended to communicate the combustion chamber and the precombustion chamber so that combustible mixture may flow into the precombustion chamber. In a variation of the invention, at least one of the let-through devices is a direct injector in the combustion chamber for, in all or in part, the fuel components and/or fuels. A method and an application are also described.

The present invention concerns an internal combustion engine withsupercharging and sparking plug with precombustion chamber, an ignitionmethod and an application of the method to the engine. It is intendedfor the industry of fuel, gas engines or others, with two or four-strokeinternal combustion, for motorised vehicles such as motorcycles,automobiles, aircrafts . . . as well as motorised apparatus such asgenerators, toolings or others. Although preferably implemented in apiston/crankshaft engine, it may be applied to a rotary type engine.

Internal combustion engines have been known for numerous years and haveevolved constantly. One considers here, as state of the art, the mostconventional internal combustion engines, i.e. with piston andcrankshaft, enabling to transform an alternate movement of a pistonunder the effect of the combustion of a combustible mixture, into acircular movement. This type of engine includes one or several enginemembers. Each engine member includes a combustion chamber, also calledcylinder, of a combustible mixture with fuel and oxidant components,generally petrol and air, and filled with a compression system, in sucha case a piston for this type of engine, an ignition system of thecombustible mixture by an electric spark generator as well as sequentiallet-through devices for the fuel and oxidant components and for thecombustion products. The operation of this type of engine, regardlesswhether two or four-stroke is known conventionally and will not bedetailed here. One considers here more particularly the superchargedengines wherein one introduces in the combustion chamber at least theoxidant components at high pressure thanks to a pressure booster. Thispressure booster is generally a turbine actuated by the exhaust gas andwhich compresses the air which must be injected in the cylinders. Othertypes of pressure boosters may be used, the turbo-compressors, thesimple compressors, the pressure wave systems, etc.

The supercharged engines require thermal protection of the boostpressure system. Indeed, by considering the case of a turbine actuatedby the exhaust gas, the former is exposed directly to the flux of hotgases coming out of the engine while the turbine shows limiteddestructive injected maximum temperature. Usually, to remedy thisshortcoming, one resorts to enrichment by addition of fuel to limit thetemperature of the gases burnt at the outlet of the engine. Indeed, thesurplus of fuel thus provided may not burn since the quantity of airpresent in the cylinder is insufficient (the quantity of air present inthe cylinder only enables to burn the fuel equivalent to the richness1), and this surplus of fuel while evaporating (latent evaporation heat)enables the load to cool down. The gases come out therefore less hotfrom the cylinder. However, this effect is proportional to the surplusof petrol provided and said petrol is only used for cooling down the gaswhich causes an increase in consumption.

For some years, it has been sought to optimise the operation of internalcombustion engines and in particular to reduce their fuel consumption aswell as the rejection of combustion products, whereas the latter hadmore and more to become ultimate combustion products and limit theoxidised nitrogen species. To do so, improvements have been introducedto the basic operation of this type of engine. Such improvements concernfor instance the ignition device, additional devices, and modalities forsupplying the oxidant and fuels components, notably with directinjection engines. Progress made in electronics and calculators havealso enabled to improve the operation of engines.

For instance, one knows from the patent EP-0831213 held byDAIMLERCHRYSLER AG, an internal combustion engine with direct injectionwhich uses a spark ignition with an ignition sparking plug. An injectorenables with high pressure to inject directly the fuel components intothe combustion chamber. The sparking plug includes simply a sleeve whichis open in the combustion chamber.

With the applications FR-2.781.840 and FR-2.810.692 one knows particulararrangements of the engine member with precombustion chamber separatedfrom the combustion chamber by a wall not letting through the flamefront generated by the ignition of a combustible mixture in theprecombustion chamber.

One knows from EP-0957246 a gas engine (CH₄) whereof the control of theignition is performed by the injection of a small quantity of liquidfuel into a precombustion chamber.

If the set of these improvements has largely enabled to reduce theconsumption and the emissions, the corresponding engines still havelimitations. In particular, the supercharged engines are penalised byexcessive fuel consumption due to enrichment.

The invention intends to provide an ignition system enabling to reduceenrichment in a supercharged engine while enabling correct combustion,i.e. with a correct combustion output, even when the mixture is preparedunder detrimental conditions. In a preferred embodiment, the ignitionsystem replaces the sparking plug on a conventional engine and nospecific arrangement to the cylinder head is necessary. The ignitionsystem includes in its portion in relation with the combustion chamber,a substantially spherical head drilled with passage holes or orifices orpassageways, these terms being equivalent. Inside said head, one findsone or several electrodes enabling to create a spark by applying avoltage therebetween.

Thus the invention concerns an internal combustion engine with at leastone engine member, the engine member including:

-   a combustion chamber of a combustible mixture with fuel and oxidant    components fitted with a compression system,-   an ignition system of the combustible mixture by an igniter,-   sequential let-through devices for the fuel and oxidant components    and for the combustion products,    the engine being of the supercharging type by boost pressure of the    oxidant components upstream of the engine member.

According to the invention, the ignition system includes a closed head,substantially spherical with a wall enclosing the igniter in aprecombustion chamber, the head including a set of orifices intended tocommunicate the combustion chamber and the precombustion chamber so thatcombustible mixture may flow into the precombustion chamber.

In various modes for implementing the invention, the following means maybe used individually or according to all the technically possiblecombinations:

-   the igniter includes a generator of electric sparks,-   the ignition system is a sparking plug with precombustion chamber,-   the ignition system includes a system for introducing the fuel and    oxidant components directly into the precombustion chamber,-   the ignition system includes an introduction device enabling direct    introduction of a combustible mixture into the precombustion    chamber,-   the separation wall between the precombustion chamber and the    combustion chamber of the head is convex outwardly of the    precombustion chamber,-   alternately, the separation wall between the precombustion chamber    and the combustion chamber of the head is concave outwardly of the    precombustion chamber,-   alternately, the separation wall between the precombustion chamber    and the combustion chamber of the head is substantially a    polyhedron, a cone,-   the sparking plug head includes a grid or porous material type wall,-   the separation wall between the precombustion chamber and the    combustion chamber of the head is made of a material with thermal    conductivity greater than 10 W/K/m,-   the separation wall between the precombustion chamber and the    combustion chamber of the head is made of a material with thermal    conductivity preferably greater than 30 W/K/m,-   the separation wall between the precombustion chamber and the    combustion chamber of the head is made of high conductivity copper    alloy,-   the copper alloy is CuCr1Zr,-   the separation wall between the precombustion chamber and the    combustion chamber of the head includes a refractory material,-   the wall of the head is covered with a substance facilitating the    reactive combustion of the fuel and oxidant components and/or the    ultimate degradation of the combustion products,-   each orifice of the precombustion chamber on the head has a diameter    smaller than or equal to 3 mm,-   the orifices of the precombustion chamber of the head are minimum    three in number,-   the orifices of the precombustion chamber of the head are arranged    on the head so that the combustion of the combustible mixture in the    precombustion chamber induces jets of matter through the orifices to    the combustion chamber distributed to ensure homogeneity of the    combustion of the combustible mixture substantially in the whole    combustion chamber,-   at least one of the let-through devices is a direct injector in the    combustion chamber for, in all or in part, the fuel and/or oxidant    components,-   the compression system is a piston in a cylindrical combustion    chamber with central axis, the injector being arranged substantially    axially opposite the piston and the ignition system laterally with    respect to the injector, and the orifices are predominantly arranged    towards the axis,-   the compression system is a piston in a cylindrical combustion    chamber with central axis, the ignition system being arranged    substantially axially opposite the piston and the injector laterally    with respect to the sparking plug, and the orifices are distributed    regularly on the surface of the head,-   the compression system is a piston in a cylindrical combustion    chamber with central axis, the injector and the ignition system    being arranged laterally with respect to said axis, and the orifices    are predominantly arranged towards the axis,-   the head of the sparking plug is arranged on a portion of the path    of the fuel components injected so that said head may be wetted by    said fuel components during the direct injection thereof,-   at least one orifice has dimensions of passageway letting through a    flame front from the precombustion chamber to the combustion    chamber,-   each orifice letting through the flame front has a diameter ranging    between 1 and 3 mm,-   at least one orifice has dimensions of passageway not letting    through a flame front from the precombustion chamber to the    combustion chamber while letting through unstable species resulting    from the combustion in the precombustion chamber in order to enable    self-ignition of the combustible mixture of the combustion chamber,    (the head may therefore include orifices of both previous types,    i.e. letting through and not letting through the flame front),-   the number of orifices enabling the propagation of a flame front    provided in the head of the precombustion chamber body varies from 1    to 5 and preferably is 1 and the number of orifices not enabling the    propagation of a flame front varies from 1 to 20, preferably from 3    to 15,-   the set of orifices has dimensions of passageway not letting through    the flame front from the precombustion chamber to the combustion    chamber while letting through unstable species,-   each orifice not letting through the flame front has a diameter    smaller than 1 mm,-   each orifice not letting through the flame front has a diameter    ranging between 0.5 and 1 mm,-   each orifice has a length smaller than its diameter,-   the engine includes moreover means enabling to re-inject with the    oxidant components a portion of the combustion products,-   the injector has multiple jets, with jets directed towards the head,-   the jets are sequentially distributed with time,-   the richness of the mixture is greater than or equal to one in at    least one embodiment of the engine.

The invention also concerns an ignition method of an internal combustionengine having at least one engine member, the engine member including:

-   a combustion chamber of a combustible mixture with fuel and oxidant    components fitted with a compression system,-   an ignition system of the combustible mixture by an igniter,-   sequential let-through devices for the fuel and oxidant components    and for the combustion products,    the engine being of the supercharging type by boost pressure of the    oxidant components upstream of the engine member.

According to the method of the invention:

-   one implements an ignition system including a closed head    substantially spherical with a wall enclosing the igniter in a    precombustion chamber, the head including a set of orifices intended    to communicate the combustion chamber and the precombustion chamber    so that combustible mixture may flow into the precombustion chamber,-   one introduces in the combustion chamber the fuel components and the    oxidant components which form the combustible mixture in the    combustion chamber,-   one causes an ignition of the combustible mixture in the    precombustion chamber by the igniter, the orifices of the    precombustion chamber enabling the ignition of the combustible    mixture of the combustion chamber.

The previous steps are also implemented in a method characterised inthat one lets through the orifices of the unstable species resultingfrom the combustion in the precombustion chamber in order to enableself-ignition of the combustible mixture of the combustion chamberwithout however letting through the flame front from the precombustionchamber to the combustion chamber.

The invention also concerns an application of the methods according tothe previous features to the engine according to one or several of thefeatures of engines listed previously, and in particular, one implementsdirect injection.

The invention finally concerns a sparking plug with precombustionchamber for implementation in the engine of the invention and accordingto one or several of the corresponding features listed previously.

The implementation of the invention in a supercharged engine enablestherefore to limit the enrichment on high load since the combustion isfaster and terminates sooner in the engine cycle. The gas derived fromthe combustion exit therefore not so hot from the cylinder since thetime between the end of combustion and the valve opening is longer. Theenrichment necessary to the protection of the turbine may therefore bereduced, hence smaller consumption on this type of supercharged enginewith respect to a conventional ignition with conventional electrode typesparking plug. The other advantages provided by the present inventionare the protection of the electrode(s) of the sparking plug. Theprobable presence of a carburized mixture in the vicinity of theelectrodes is higher.

Moreover, the ignition system with precombustion chamber enables toobtain faster combustion, even in the case of little homogeneousmixture. Indeed, on the one hand several flame fronts reach differentzones of the combustion chamber and on the other hand, the radicalsemitted by the orifices of the igniter seed the combustion chamber indifferent points and under the combined effects of the pressure and ofthe temperature generated by the rising piston, these precursors ignitethe mixture in different points of the combustion chamber. Theprobability of having precursors in a zone favourable to the initiationof the combustion is therefore much higher than in the case of aconventional sparking plug. Moreover, the wall of the precombustionchamber protects the electrodes from the impact of liquid fuel thereon,hence a better behaviour on cold start and in reducing the soiling ofthe sparking plug. Then, in a particular embodiment, it is possible touse a jet of injector wetting directly the walls of the precombustionchamber, which causes the carburized mixture to rise inside theprecombustion chamber. This may have beneficial effects on the start andthe initiation generally.

Finally, in the case of the layered combustion, the injection of fuel onthe hot head of the igniter enables to increase the AI/AA robustness byvaporising the fuel close to the electrodes (AI corresponds to theinjection advance and AA to the ignition advance).

The invention may thus be implemented advantageously in the case ofengines with direct injection of petrol or of air-petrol mixture as willbe seen below in detail. The direct injection may indeed concern eitherindividual fuels generally under high pressure of 100 bars, or apre-mixture of fuels/oxidants generally under low pressure, of the orderof 5 to 10 bars.

The present invention will now be exemplified with the followingdescription and in relation with:

FIG. 1 which represents, seen from the precombustion chamber, theprogression of the combustion produced by a spark,

FIG. 2 which represents, seen from the combustion chamber, the differentorifices of the sparking plug head,

FIGS. 3, 4, 5, 6 which represent as a sectional view, a cylinder fordifferent operating phases of an engine according to the invention inthe case of direct injection,

FIG. 7 which represents an example of embodiment of an ignition systemwith a sparking plug, as a partial sectional view,

FIG. 8 which represents an example of embodiment of a head of theignition system.

The ignition systems whereof the igniter is a discharger intended toproduce electric sparks, of the type sparking plug, may have differentconfigurations in relation to the type of engine and/or to theperformances desired. For instance the sparking plugs may be of variouslengths. The invention which implements a sparking plug withprecombustion chamber may employ different configurations of sparkingplugs and a particular example will be given at the end of thedescription. In the invention, the sparking plug includes aprecombustion chamber according to features which will now be detailed.

Experiments have been carried out on several configurations of ignitionsystems according to the invention. The features of the sparking plugwith precombustion chamber more particularly assessed are:

-   Volume: 700 mm³-   Section of passage: 5.1 mm²-   S/V ratio (mm⁻¹): 7.4×10⁻³ mm⁻¹    (S being the sum of the sections of the passageways and V the volume    of the precombustion chamber)-   Inter-electrode distance: 0.7 mm.

As regards the S/V ratio, it is a quality indicator. The lower thisratio, the better the output of use of the oxidant and of fuel. Theoptimum case is that of the spherical precombustion chamber.

The ignition system of the sparking plug type with precombustion chamberof the invention is a component which does not require any particularmachining of the engine. The implantation may take place in aconventional engine sparking plug well since its diameter may be smallerthan or equal to 14 mm. The volume of the precombustion chamber mayrange between 0.2 and 2 cm³. Preferably, the precombustion chamber has avolume smaller than 1.5 cm³, generally ranging between 0.5 cm³ and 1.5cm³. Generally, the ratio between the volume of the precombustionchamber and the dead volume of the main chamber varies between 0.1 and5%, preferably between 0.1 and 2%. The form of the head of the ignitionsystem is preferably a spherical cap.

Optionally, the ignition system may moreover include an intake enablingto supply directly the precombustion chamber with a combustible mixtureformed upstream or to introduce fuel, the air being then mixed with thefuel in the precombustion chamber.

The ignition system includes in its portion in relation with thecombustion chamber, a spherical head drilled with holes or orifices orpassageways. These terms being equivalent in the context of theinvention. Inside said head, one finds the igniter in the form of one orseveral electrodes enabling to create a spark by applying a voltagetherebetween.

When developing the ignition system, several configurations of materialhave been tested for the wall of the precombustion chamber, i.e.: Steel35CD4; alloy Ni—Fe—Cr, copper alloy whereof brass or copper-nickel-zincalloys or with nickel or aluminium; high conductivity copper alloyCuCr₁Zr. The best results have been obtained with the latter material.The alloy CuCr₁Zr is a grade of the alloy CRM16x with a ratedcomposition Cr>0.4%, Zr from 0.022% to 0.1% and the remainder is copper.

As stated above, the precombustion chamber may be made of a materialhaving a thermal conductivity greater than 10 W/K/m and preferablygreater than 30 W/K/m. One may thus use materials whereof the thermalconductivity may reach 350 W/K/m. The use of such a material, preferablya copper alloy, enables to evacuate the energy at the wall ofprecombustion chamber and thus to make up for the occurrence of hotpoints at the precombustion chamber. For instance, for the materialsubject to in-depth experiments, the alloy CuCr1Zr, the thermalconductivity at 20° C. of 320 W/K/m.

For exemplification purposes for other copper-based usable materials,one may consider a grade of brass: CUZn37 of conductivity: 113 W/K/m.

The following table shows different binary brasses, copper-nickels,copper-aluminium and copper-nickel-zinc alloys being usable as materialfor the precombustion chamber body. The level of thermal conductivity aswell as the mechanical handling at high temperature (450-1000 K)determines the selection of the material.

Brass: Physical Properties Property Cu Zn 5 Cu Zn 10 Cu Zn 15 Cu Zn 20Cu Zn 30 Cu Zn 33 Cu Zn 36 Cu Zn 40 (composition according to thestandard NF A 51- 101) Thermal conductivity at 234 188 159 138 121 117117 121 20° C. [W/(m · K)]

Copper-Nickel-Zinc Alloys: Physical Properties Property Cu Ni 10 Cu Ni12 Cu Ni 15 Cu Ni 18 Cu Ni 18 Cu Ni 10 Cu Ni 18 Zn (compositionaccording Zn 27 Zn 24 Zn 21 Zn 20 Zn 27 Zn 42 Pb 2 19 Pb 1 to thestandard NF A 51-101) Thermal conductivity at 38 38 34 29 25 34 25 20°C. [W/(m · K)]And finally the alloys:

-   Copper-nickel: 21 W/(m·K) (Cu Ni 44 Mn) to 63 W/(m·K) (Cu Ni 5 Fe)-   Copper-aluminium: 75 to 84 W/(m·K) (Cu Al 5, Cu Al 6), 38 to 46    W/(m·K) (Cu Al 10 Fe 5 Ni 5)-   These data are derived from “Technique de l'Ingénieur”, Volume    MB5—Etude et propriétés des métaux M 437

This type of sparking plug with precombustion chamber is used preferablywith an engine showing optimised permeability of the cylinder head tothe detriment of the aerodynamics of the combustion chamber. Indeed, thecombustion mode resulting from the use of the sparking plug withprecombustion chamber enables sufficient combustion speed to dispensewith an increase in the combustion speed via the aerodynamics of thecombustion chamber.

The invention may be implemented as well on a conventional superchargedengine wherein one introduces a combustible mixture in the combustionchamber, i.e. that the mixture has taken place upstream of the enginemember, as on an engine with direct injection of the fuel components.

Engine tests for assessing the potential of sparking plugs withprecombustion chamber have been conducted on heavily superchargedengines, i.e. up to more than 15 bars. These tests have enabled to putin evidence a path of improvement regarding full load enrichment(PME=efficient average pressure=13 bars) when using sparking plugs withprecombustion chamber.

One has shown during experiments on this type of supercharged enginenotably the following improvements: a reduction in full load enrichment,total or partial inhibition, pinkling phenomenon with a volumetric ratioranging between 8 and 14, better use of the air.

FIG. 1 represents therefore, as seen in the precombustion chamber 1, theprogression of the combustion 2 generated by a spark 3.

FIG. 2 represents therefore, as seen in the combustion chamber 4, thedifferent orifices 5 of the wall of the head 6 of sparking plug enablingcommunications between the precombustion chamber 1 and the combustionchamber 4, also called main chamber. Thanks to these communications, onthe one hand, combustible mixture flows from the combustion chambertowards the precombustion chamber and, on the other hand, after ignitionin the precombustion chamber, the combustible mixture of the combustionchamber may finally ignite. The disposition of orifices enablessubstantially homogeneous distribution of the flame front and/or of theunstable species which enable the ignition of the combustible mixture ofthe combustion chamber.

Preferably the sparking plug and its head are a single component whichreplaces a traditional sparking plug which does not require anymodifications of the cylinder head passageway for the sparking plug. Theignition system is thus formed of a device replacing the conventionalsparking plug. One also contemplates that the system generating thespark be modified in relation to the form of the head and, for instancethat the central electrode moves forward further in the head and comescloser to the wall of the former so that the electric arc is formedbetween the central electrode and the wall of the head. It should beunderstood that in such a case, the wall must include a conductivematerial of the electricity for the spark current to return to theground. One also contemplates that the head of sparking plug be aremovable part, for instance by screwing, and which may be unscrewed togain access to the electrode(s) of the sparking plug for possibleadjustment of the gap or inspection. In the latter case, it may bedesirable that the head extends laterally towards the rear in a screwingzone on the cylinder head to be held and may not be unscrewed nor fallin the combustion chamber because of the vibrations of the engine.

It should be understood that the examples given are purely illustrativeand that the invention may be varied according to diverse possibilities.It has been observed thus that a head with orifices of both types couldbe used, i.e. letting through and not letting through the flame front.Similarly, the orientations orifices may be optimised in relation to therelative disposition of the different members in the engine.

Thus, among the applications of the invention one may quote the engineswith direct injection of the fuel components. Indeed, apart from theinvention, the engines with direct injection are generally penalisedwhen preparing the mixture with respect to the injection engines ininlet conduits by problems of homogeneity of the mixture in thecombustion chamber. Similarly, the engines with direct injectionexhibit, in certain configurations of arrangement of the injector and ofthe sparking plug, problems of direct impact of the fuel on theelectrodes of the sparking plug, hence problems of cold start and ofsoiling of electrodes of the sparking plug. Finally, the engines withdirect injection are sensitive to the impact of the relatively cold fuelon the walls of the cylinder. Still, for correct initiation (ignition),it may be necessary to have sufficient jet to come close to the sparkingplug, which implies high walls effect.

The implementation of the invention, thanks to the head of the ignitionsystem which encapsulates the electrode(s) of the sparking plug, enablesto limit these shortcomings. FIGS. 3, 4, 5, 6 represent thus aparticular implementation of the invention with an engine member withdirect injection seen as an axial section of a piston 9 and goingthrough a injector 8 and a sparking plug 7. The other inlet and exhaustmembers for sequential passageways of the oxidant components (possiblyfuel) and for the combustion products, are not detailed therein. On FIG.3, the phase of injection of the fuel components by the injector 8 is inprogress and a portion of the fuels will wet the head 6 of the sparkingplug 7 which surrounds the precombustion chamber. On FIG. 4, the enginebeing hot, the fuels are vaporised simultaneously on the piston 9 whichincludes a <<piston bowl>> and on the head 6 of the sparking plug 7. OnFIG. 5, the phase of compression has begun and combustible mixture flowsfrom the combustion chamber 4 towards the precombustion chamber 1 of thesparking plug 7. On FIG. 6, the phase of combustion has begun from theprecombustion chamber 1 wherein a spark has been produced and by passingthrough the orifices 5 of the head 6, the flame front and/or accordingto the type of orifice, the unstable species for propagation to thecombustion chamber 4.

As stated above previously, the implementation of the invention enablesreduction in the pinking, this being notably perceivable in the case ofthe operation of the engine on high load. To give an order of magnitude,by low load operation of the engine is meant the operating range of theengine from idle to a quarter of the full load of the engine, preferablythe range from idle to 10% of the full load in the case of anatmospheric engine and the range from idle to 5% of the full load in thecase of an heavily supercharged engine.

In an alternate embodiment of the head of the ignition system, oneimplements orifices in the wall of the head which have a differentialeffect, letting through or not letting through the flame front inrelation to the load of the engine. This effect may be obtained forinstance by at least one orifice enabling the propagation of a flamefront on low load and at least one orifice not enabling the propagationof a flame front under any load conditions or, then, through simply atleast one orifice enabling the propagation of a flame front on low load.In the case of a low load operation of the engine, the flame front mayflow from the precombustion chamber to the main combustion chamber bymeans of the passageway(s) enabling the propagation of a flame front andone causes thus the ignition of the main combustible mixture via a flamefront. In the case of an operation on high load of the engine, thestructure orifices which let through the flame front on low load is suchthat, on high load, the flame front does not flow any longer, saidorifices causing an extinction of the flame front and these are then theunstable compounds from the combustion of the combustible mixture to theprecombustion chamber which induce mass self-ignition of the maincombustible mixture seeded with the unstable compounds in the mainchamber.

One may explain this phenomenon as follows. Taking into account thesmall quantity of air/fuel mixture in the precombustion chamber in thecase of low supercharge, the pressure rise in the precombustion chamberat the ignition is significantly less violent than in the case of highloads and the flame front obtained by the combustion of the mixture inthe precombustion chamber may, thanks to the passageway of largediameter propagate in the main combustion chamber. Said continuouspropagation of the flame front between precombustion chamber and maincombustion chamber causes stability on low load similar to theconventional case of the controlled ignition engines. In the case ofhigh loads, the quantity of carburized mixture in the precombustionchamber is from 3 to 7 times greater than the little supercharged case.Consequently, during the combustion of the mixture in the precombustionchamber, the pressure rise is significantly greater. The differentialpressure between the precombustion chamber and the main chamber preventsthe propagation of the flame front from the precombustion chamber to themain chamber. Nevertheless, the passageways let flow the flux ofunstable compounds from the precombustion chamber to the main chamberand, during the rising of the piston, the compression produces massself-ignition of the main mixture.

It should be understood that one may also use orifices letting throughor not letting through the flame front regardless whether under allconditions or not to obtain differential effects said times in volume,certain types of orifices being arranged on the head preferably to havecombustion-triggering effects in a more particular zone of thecombustion chamber. These differential effects in relation to the loadand to the volume may be combined by a particular arrangement oforifices of different structures on the head.

In another alternative embodiment of the head of the ignition system,possibly combined to the latter, the internal and/or external faces ofthe wall of the head of the precombustion chamber as well as, possibly,the walls of the orifices, are coated with a refractory coating layer,such as, for instance, coating layers en Al₂O₃, ZrY (not necessarilystoichiometric) and TiB₂. The thickness of these coating layers rangesgenerally between 5 and 100 μm, preferably 1 to 50 μm. One increasesthus the efficiency of combustion in the precombustion chamber andimproves the low load operation, in particular in the case of heavilyoverboosted engines.

As stated above, different configurations of sparking plugs withprecombustion chamber may be used. There is given on FIG. 7 a particularexample of sparking plug with precombustion chamber which includes abody in the extension of the head. The sparking plug 7 is screwed in athread 10 a of the cylinder head 10 closing a cylinder of an internalcombustion engine whereof only a portion has been represented. Thesparking plug 7 includes a body 12 of precombustion chamber 1 generallytubular in shape and comprising a head 12 a generally convex in shape,preferably having the form of a spherical cap. The head 12 a of the body12 of precombustion chamber 1 forms a separation wall between thecombustion chamber 4 and the precombustion chamber 1. The precombustionchamber 1 includes the igniter comprising a central electrode 13 and aground electrode 14. The head 12 a forming the separation wall betweenthe combustion chamber 4 and the precombustion chamber 1 is providedwith different let-through orifices 5, represented more in detail onFIG. 8.

In this example, the orifices 5, generally cylindrical in shape,comprise a passageway 5 a, having a large diameter, i.e. a diametergreater than 1 mm, generally ranging between more than 1 mm and 3 mm anda series of orifices 5 b to 5 i (7 in the embodiment of FIG. 8) having asmall diameter, i.e. smaller than or equal to 1 mm. Generally, thelength of the passageways is smaller than 1 mm and they are hereoriented according to radii of the hemispherical head 12 a. One has alsoseen that the orientation and/or the structure of orifices may be suitedto the particular configuration of the element of the engine.

Although one has represented a single orifice 5 a of diameter greaterthan 1 mm (preferred embodiment), the head 12 a may include severalorifices of large diameter. However in certain applications, as alreadyseen with the differential effects orifices on the flame front inrelation to the load, the number and the dimension orifices must be suchthat no flame front may propagate from the precombustion chamber to thecombustion chamber when the engine operates on high load.

It should be understood that the examples given are purely illustrativeand that it is possible to implement the invention according to diversemodalities without departing from its general framework.

1. An internal combustion engine with at least one engine member, theengine member including: a combustion chamber of a combustible mixturewith fuel and oxidant components fitted with a compression system, anignition system of the combustible mixture by an igniter, sequentiallet-through devices for the fuel and oxidant components and for thecombustion products, the engine being of the supercharging type by boostpressure of the oxidant components upstream of the engine member,wherein the fuel is exclusively liquid, notably petrol, and the ignitionsystem includes a closed head substantially spherical with a wallenclosing the igniter in a precombustion chamber, the head including aset of orifices intended to communicate the combustion chamber and theprecombustion chamber so that combustible mixture may flow into theprecombustion chamber.
 2. An engine according to claim 1, wherein atleast one orifice has dimensions of passageway letting through a flamefront from the precombustion chamber to the combustion chamber.
 3. Anengine according to the claim 1, wherein at least one orifice hasdimensions of passageway not letting through a flame front from theprecombustion chamber to the combustion chamber while letting throughunstable species resulting from the combustion in the precombustionchamber in order to enable self-ignition of the combustible mixture ofthe combustion chamber.
 4. An engine according to claim 3, wherein theset of orifices have dimensions of passageway not letting through theflame front from the precombustion chamber to the combustion chamberwhile letting through unstable species.
 5. An engine according to claim3, wherein each orifice not letting through the flame front has adiameter smaller than 1 mm.
 6. An engine according to claim 3, whereineach orifice has a length smaller than its diameter.
 7. An engineaccording to claim 1, wherein the separation wall between theprecombustion chamber and the combustion chamber of the head is made ofa material with thermal conductivity greater than 10 W/K/m.
 8. An engineaccording to claim 1, wherein the separation wall between theprecombustion chamber and the combustion chamber of the head is made ofhigh conductivity copper alloy (CuCr1Zr).
 9. An engine according toclaim 1, wherein the orifices of the precombustion chamber of the headare minimum three in number.
 10. An engine according to claim 1, whereinit is with direct injection of the fuel components in the combustionchamber.
 11. An engine according to claim 10, wherein the compressionsystem is a piston in a cylindrical combustion chamber with centralaxis, at least one of the let-through devices being a direct injector inthe combustion chamber for, in all or in part, the fuel and/or oxidantcomponents, the injector being arranged substantially axially oppositethe piston and the ignition system laterally with respect to theinjector, and the orifices are predominantly arranged axially to ensurehomogeneity of the combustion of the combustible mixture substantiallyin the whole combustion chamber.
 12. An engine according to claim 1,wherein the compression system is a piston in a cylindrical combustionchamber with central axis, at least one of the let-through devices beinga direct injector in the combustion chamber for, in all or in part, thefuel and/or oxidant components, the ignition system being arrangedsubstantially axially opposite the piston and the injector laterallywith respect to the ignition system, and the orifices are distributedregularly on the surface of the head to ensure homogeneity of thecombustion of the combustible mixture substantially in the wholecombustion chamber.
 13. An engine according to claim 1, wherein thecompression system is a piston in a cylindrical combustion chamber withcentral axis, at least one of the let-through devices being a directinjector in the combustion chamber for, in all or in part, the fueland/or oxidant components, the injector and the ignition system beingarranged laterally with respect to said axis, and the orifices arepredominantly arranged axially to ensure homogeneity of the combustionof the combustible mixture substantially in the whole combustionchamber.
 14. An engine according to claim 1, wherein the head isarranged on a portion of the path of the fuel components injected sothat said head may be wetted by said fuel components during the directinjection thereof.
 15. An engine according to claim 10, wherein therichness of the mixture is greater than or equal to one in at least oneembodiment of the engine.
 16. An engine according to claim 1, whereinthe ignition system and its head are a single component which replaces atraditional sparking plug and which does not require any modification ofthe cylinder head passageway for the sparking plug.
 17. A method ofignition of an internal combustion engine having at least one enginemember, the engine member including: a combustion chamber of acombustible mixture with fuel and oxidant components fitted with acompression system, an ignition system of the combustible mixture by anigniter, sequential let-through devices for the fuel and oxidantcomponents and for the combustion products, the engine being of thesupercharging type by boost pressure of the oxidant components upstreamof the engine member, wherein one implements an ignition systemincluding a closed head substantially spherical with a wall enclosingthe igniter in a precombustion chamber, the head including a set oforifices intended to communicate the combustion chamber and theprecombustion chamber so that combustible mixture may flow into theprecombustion chamber, one introduces in the combustion chamber the fuelcomponents and the oxidant components which form the combustible mixturein the combustion chamber, the fuel being exclusively liquid, notablypetrol, one causes an ignition of the combustible mixture in theprecombustion chamber by the igniter, the orifices of the precombustionchamber enabling the ignition of the combustible mixture of thecombustion chamber.
 18. A method according to claim 17, wherein one letsthrough the orifices of the unstable species resulting from thecombustion in the precombustion chamber in order to enable self-ignitionof the combustible mixture of the combustion chamber without howeverletting through the flame front from the precombustion chamber to thecombustion chamber.
 19. A method according to claim 17, wherein theengine is an internal combustion engine with at least one engine member,the engine member including: a combustion chamber of a combustiblemixture with fuel and oxidant components fitted with a compressionsystem, an ignition system of the combustible mixture by an igniter,sequential let-through devices for the fuel and oxidant components andfor the combustion products, the engine being of the supercharging typeby boost pressure of the oxidant components upstream of the enginemember, wherein the fuel is exclusively liquid, notably petrol, and theignition system includes a closed head substantially spherical with awall enclosing the igniter in a precombustion chamber, the headincluding a set of orifices intended to communicate the combustionchamber and the precombustion chamber so that combustible mixture mayflow into the precombustion chamber.